One technology for after-treatment of engine exhaust utilizes selective catalytic reduction (SCR) to enable certain chemical reactions to occur between NOx in the exhaust and ammonia (NH3). NH3 is introduced into an engine exhaust system upstream of an SCR catalyst by injecting urea into an exhaust pathway. The urea entropically decomposes to NH3 under high temperature conditions. The SCR facilitates the reaction between NH3 and NOx to convert NOx into nitrogen (N2) and water (H2O). However, as recognized by the inventor herein, issues may arise upon injecting urea into the exhaust pathway. In one example, urea may be poorly mixed into the exhaust flow (e.g., a first portion of exhaust flow has a higher concentration of urea than a second portion of exhaust flow) which may lead to poor coating of the SCR and poor reactivity between emissions (e.g., NOx) and the SCR. Additionally, overly mixing and agitating the urea in the exhaust can likewise cause issues, such as increased deposits.
Attempts to address insufficient mixing include introducing a mixing device downstream of a urea injector and upstream of the SCR such that the exhaust flow may be homogenous. Other attempts to address urea mixing include a stationary mixing apparatus. One example approach is shown by Cho et al. in U.S. 2013/0104531. Therein, a static mixer is located in an exhaust passage downstream of an external tube for injecting urea. The exhaust gas flows through the exhaust passage and combines with a urea injection before flowing through the static mixer.
However, the inventors herein have recognized potential issues with such systems. As one example, the static mixer described above presents limited mixing capabilities due to a directionality of exhaust outflow through the mixer unable to fully mix a laminar exhaust flow. The static mixer inside the exhaust passage also presents manufacturing and packaging constraints. Varying exhaust passage geometries demand an alteration in the manufacturing of the static mixer for the mixer to tightly fit within the exhaust passage.
In one example, the issues described above may be addressed by a system for a urea injector injecting urea to a perforated tube, the tube is a toroid and configured to receive exhaust gas with inlets located on an upstream face facing a direction of incoming exhaust flow in an exhaust passage. In this way, urea may mix with exhaust gas in the perforated tube before entering the exhaust passage.
As one example, the perforated tube further comprises inner and outer outlets facing a direction perpendicular to incoming exhaust flow. Additionally, the inner and outer outlets face a central region and an outer region of the exhaust passage. The urea may mix with exhaust gas in the perforated tube before flowing out either of the inner and outer outlets. A mixture flowing out of the inner outlets flows in a radial inward direction to the central region and a mixture flowing out of the outer outlets flows in a radial outward direction to the outer region. In this way, an entire exhaust flow through the exhaust passage may come into contact with urea and increase mixing.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.